The present invention relates to a chamber for use in a fumigation method to produce a fumigant gas mixture and supply it to a circulatory loop gas flow system including an enclosed fumigation space wherein fumigation of a commodity is to take place and a gas duct including gas propulsion device connected to opposite regions of the fumigation space, the chamber being adapted to receive fumigant gas generation means for releasing fumigant gas. The invention also relates to a method of fumigating a bulk commodity in an enclosed fumigation space and to the use of the chamber in such a method.
Phosphine gas is a highly toxic and flammable gas used in large quantities more often than not as the fumigant gas of choice in pest control, and in particular for the fumigation of agricultural bulk commodities, such as grain and grain products.
Traditionally compositions containing hydrolyzable metal phosphides, in particular aluminum, magnesium and calcium phosphides have been used for this purpose, applied either in sachets or other dispenser devices or as moulded bodies (pellets or tablets).
In either case, the traditional compositions have always been compounded with various additives to a) reduce the reactivity of the metal phosphide when exposed to water in vapor or liquid form and b) to depress their tendency to autoignite. (Rauscher et al U.S. Pat. No. 3,132,067, Friemel et al U.S. Pat. No., 3,372,088, Friemel et al U.S. Pat. Nos. 4,421,742 and 4,725,418, Kapp U.S. Pat. No. 4,347,241). In spite of these expedients, these prior art products remained dangerous substances, involving fire and explosion hazards which had never been fully overcome if the products are handled inexpertly and stringent safety precautions are neglected. The degree of safety also depends on the experience of the manufacturer and quality control.
Specific methods relate to the fumigation of bulk commodities in bulk storage containers, such as silo bins and ship holds. Formerly the usual method involved introducing metal phosphide compositions in the form of pressed bodies, for example pellets or tablets, simultaneously with the bulk commodity, more or less evenly distributed therein. A disadvantage of this method resides in the unavoidable contamination of the bulk commodity with the residues of the spent pest control compositions.
A more recent, commercially very successful, method is to distribute fumigant gas generation means such as the aforesaid pellets or tablets on the surface of the product to be treated and thereafter to ensure a circulation of air within the enclosed environment in order to effect uniform distribution of the gas and avoid the buildup of dangerously high local concentrations of the gas which could lead to autoignition and comatose states of the pests from which the latter may subsequently recover. (U.S. Pat. Nos. 4,200,657 and 4,651,463 corresponding to ZA Patents 79/6807 and 83/5402 and corresponding patents in other countries) In order to avoid the above-mentioned contaminations, the gas generation means may be sachets containing hydrolyzable metal phosphides and/or what is known in the art as "bag blankets" containing same which are suspended or rolled over the surface of the product to be treated.
These latter systems, employed up until now, had been a great improvement but have still suffered from a number of disadvantages.
Firstly, a distribution of the gas generation means on the surface of the product to be disinfested results in the gas generation means needing to be collected after the fumigation process is completed. This is often difficult and cumbersome. The spent devices must then be disposed of, a matter which nowadays may cause problems.
Also, an equal distribution of the gas generation means on the surface of the commodity to be fumigated needs to be achieved. The phosphine gas can, over a certain concentration (17 800 ppm), become self-igniting. An accumulation of gas generation means in a particular area may lead to dangerous concentrations of this phosphine gas forming within the closed environment, unless the prior art precautions, such as adequate circulation, are strictly observed, as may happen if the circulation is too slow or is inadvertently stopped, for example, due to a power failure. Excessive localized phosphine concentrations may also temporarily put the pests into a comatose state from which they may recover, causing incomplete pest control.
Coupled to this, air flow through a conical shape (as grain etc tends to assume in a silo) is irregular, there being a lower air flow through the apex of the cone. This may result in a concentration of phosphine gasses in this area and general lack of uniformity of the gas distribution and circulation, a matter aggravated very often in applying gas circulation to ship hold fumigation.
Further to this, the conditions within the confined environment cannot easily be controlled. The silos etc are normally exposed to the elements, sun, rain etc., which may affect the rate of gas generation. In addition, agricultural products are known to respire and the relative humidity content may change over a period of time. Should the gas generation means come into contact with liquid water, the phosphine gas generation reaction may become too violent.
Finally, once the fumigation process is initiated, it is difficult and usually impossible to control the rate of gas generation or to stop the fumigation process.
The aforesaid U.S. Pat. No. 4,200,657 does disclose an embodiment wherein the phosphide pellets or tablets are laid out in a gas distribution chamber within the circulation pipe system near the bottom of a silo bin. Although this embodiment avoids direct contact of the composition with the bulk commodity and thus avoids the contamination, it does not address the remaining problems. In particular it can neither prevent dangerous localized accumulations of phosphide composition and localized gas and heat build-up, nor can it be used to stop the phosphine releasing reaction in the event of an emergency.
Accordingly, it has been recognized that it would be highly advantageous if it were possible to transfer the generation of phosphine gas to a locality outside the fumigation space whereafter the gas could then be fed into the commodity or storage facility in a controlled manner. However, because of the conceived and real risks inherent in phosphine gas and phosphine-releasing compositions, very, little real progress has been made in this regard.
Thus the use of bottled PH.sub.3, produced by one or other undisclosed industrial process, has been proposed in U.S. Pat. No. 4,889,708. Again, in order to prevent autoignition once the gas is released into air and the mixture of air and gas is used as a fumigant, it was considered necessary to bottle the PH.sub.3 highly diluted with an inert carrier gas such as CO.sub.2 or N.sub.2. According to U.S. Pat. No. 4,889,708 the PH.sub.3 concentration in the bottled gas is to be 1.8 to 3% by weight. The storage and transport of this highly diluted phosphine gas involves considerable logistics problems, besides being very expensive. It also involves the grave risk that in the event of an accident on site, in transport or in storage or in the event of leaking bottles, e.g., due to defective or not properly closed valves, a gas cloud, albeit not readily flammable, is formed which is highly toxic and which, because it is heavier than air, can accumulate in low-lying areas or in cellars or the like.
U.S. Pat. No. 5,098,664 (RSA Patent No 88/8881) and corresponding foreign patents provide an apparatus and process for generating a mixture of phosphine gas and air for fumigating an enclosed environment, the gas generation process being effected separate from the enclosed environment, the gas/air mixture thereafter being fed into and optionally circulated through the enclosed environment and product to lie disinfested. The rate of gas generation is controlled by supplying an air supply to the gas generation means, the air supply having a controlled humidity level and temperature.
In the event of an operational failure, for example a power failure, it is possible to stop the further generation of phosphine gas by flushing the generator space with an inert flushing medium. As disclosed in this patent the volume of air in which the phosphine generation takes place constitutes hut a small fraction of the total volume of the fumigating space into which the fumigation takes place. Where the latter volume is circulated, the generator air circuit constitutes a minor side stream to the main circulatory system. The disclosed generator is not designed to handle very large circulation volumes, and it would have been unduly costly to control the moisture content of the entire volume of air of the fumigation space in the manner there disclosed. As it is, the control means required for this prior art is intricate and expensive.
A number of more recent similar proposals are disclosed in PCT application WO 91/19671. Some embodiments again involve reaction of metal phosphide compositions with water vapor, and these embodiments are subject to the abovementioned problems. In most embodiments the phosphine is released into a small stream of air, and the risk of ignitable mixtures of phosphine and air being formed cannot be excluded.